There have long been trials in operating motor vehicles with fuel containing Ethanol (Alcohol) and in South America and North America in particular many motor vehicles are equipped for such operation. The concentration of the Ethanol in the fuel in the tank can vary from filling to filling depending on the type of fuel put into the tank. In such cases any Ethanol content of the fuel between 0 and 100% can be produced. For trouble-free operation of the internal combustion engine it is necessary for the engine management device to detect the new fuel composition as soon as possible and take it into account in its regulation strategies.
In the prior art (see U.S. Pat. No. 6,257,174 B1 for example) it is already known that the different air requirement of Ethanol on the one hand and other fuels on the other hand (Ethane 8,9; Gasoline: 14,7) can be used to determine the Ethanol content of the fuel. When the Ethanol content of the fuel changes, the excess-air ratio λ measured by the λ sensor, and from this change in the excess-air ratio the Ethanol proportion of the fuel can be deduced (λ method).
However the λ method only enables the Ethanol content to be determined reliably when the fuel system is intact. Since the Fuel System Diagnosis (FSD) is likewise conducted by means of the λ values measured by the λ sensor, the λ method does not provide any reliable λ values. The engine management device can namely not establish uniquely whether a deviation of the λ value is the result of a changed Ethanol content of the fuel or of a fault in the fuel system.